The invention relates to a heated roll. Such a heated roll comprises a circular-cylindrical casing tube made form a thermally conductive metal, which is closed with caps at its end faces, one or more electric heating mats, which are arranged inside the casing tube, a connecting pin, which is arranged at a cap, extending along the axis of the roll and away from the casing tube, and which shows an axial passage for a connection cable to supply the heating mats with electric power, as well as an annular brushing body, which is fastened at the connecting pin and serves for the electric contacting of the connection cable.
Heated rolls of the present type are used in various arrangements, for example in drum driers for paper webs and the like as well as in copying devices. The preferred field of application of the present invention, with the invention not being limited thereto, though, is the application of hot-melt adhesives onto planar work pieces, for example aluminum plates or planar, engineered wood materials, which shall be coated with wood veneer, a film, or other web-shaped materials.
In prior art, heated rolls are heated with a thermic oil, which flows through the interior of the roll or fills the interior of the roll, and which is heated by electric heating cartridges, arranged approximately in the center of the roll. The use of thermic oil is however not optimal, because here a volume changing connection must be fastened at the roll with appropriate tubing. Additionally, the roll must be expensively protected from oil seeping out.
In order to avoid the use of thermic oil, sometimes heating cartridges are used inside the roll, without filling it with oil. However, this leads to a very uneven heat distribution on the exterior surface of the casing tube. Other alternatives known include the use of infrared radiators, which are arranged inside or outside the roll and which heat the casing tube. In addition to the high construction expense, these alternatives also exhibit problems, such as poor effectiveness and the risk of a local overheating in case of malfunctions during operation, for example when the arrangement is stationary, in which the roll is used.
Accordingly it has been suggested in DE 30 33 689 A1 to equip a heated roll with an inner tube, arranged coaxially inside the casing tube, made from a thermally conductive metal, and to insert electrical heating mats into the annular clearance formed between the casing tube and the inner tube, which heat the casing tube from the inside to the desired temperature. According to prior art, here silicon mats with embedded heating wires are provided as electric heating mats, with it being suggested for a temperature distribution as evenly as possible over the entire circumference of the casing tube to distribute several flat abutting heating mats over the circumference of the annual clearance.
In the preferred field of application of the present invention it is of the utmost importance that the exterior surface of the casing tube is at a very even temperature.
An arrangement for applying hot-melt adhesive generally includes an application roll for applying the hot-melt adhesive onto the surface of a work piece to be provided therewith as well as a dosing roll, which together with the application roll forms a dosing gap, in which either already molten and thus hot supplied hot-melt adhesive is inserted or into which the hot-melt adhesive is supplied in a solid form and melted on site. In both cases, the dosing roll must be heated, in order to allow melting the hot-melt adhesive and/or keeping it at a temperature, which renders it liquid for application and/or within a certain range of viscosity.
Commercial hot-melt adhesives largely change their dynamic viscosity in case of temperature changes; typical values are here changes in viscosity by up to 10,000 mPas at a temperature change by approximately 10° C. When the viscosity varies for example along the longitudinal extension of the application roll, the application roll and/or dosing roll is unevenly deformed in the dosing gap such that the hot-melt adhesive in the dosing gap is dosed with an uneven distribution on the application roll. This leads to an uneven application pattern on the work piece, which is not optimal with respect to quality.